This invention relates to coated stents for carrying biologically active agents to provide localized treatment at the implant site and methods of applying stent coatings. In particular, this invention relates to antithrombogenic and antirestenotic stents having a multi-layered coating, wherein the first or inner layer is formed from a polymer and one or more biologically active agents, and a second or outer layer is formed from an antithrombogenic heparinized polymer. This invention also relates to methods of applying a multi-layer coating over the surface of a stent and methods of using such a coated stent.
Atherogenic arterial narrowing and thrombosis are two potentially fatal, related conditions that have been identified as leading killers by various health organizations in the United States and throughout the world. Stenosis refers to the narrowing or constriction of a vessel, which is usually due to the buildup of fat, cholesterol, and other substances over time. In severe cases, stenosis can completely clog a vessel. Thrombosis is the formation or presence of a blood clot inside a blood vessel or cavity of the heart. The clot is usually formed by an aggregation of blood factors, primarily platelets and fibrin, with entrapment of cellular elements. Thrombosis, like stenosis, frequently causes vascular obstruction at the point of its formation.
One approach to clearing an artery that has been constricted or clogged due to stenosis is percutaneous transluminal coronary angioplasty (PTCA) or balloon coronary angioplasty. In this procedure, a balloon catheter is inserted and expanded in the constricted portion of the vessel for clearing the blockage. About one-third of patients who undergo PTCA suffer from restenosis, the renarrowing of the widened segment, within about six months of the procedure. Restenosed arteries may have to undergo another angioplasty.
Restenosis can be inhibited by a common procedure that consists of inserting a stent into the effected region of the artery instead of, or along with, angioplasty. A stent is a tube made of metal or plastic, which can have either solid walls or mesh walls. Most stents in use are metallic and are either self-expanding or balloon-expandable. The decision to undergo a stent insertion procedure depends on certain features of the arterial stenosis. These include the size of the artery and the location of the stenosis. The function of the stent is to buttress the artery that has recently been widened using angioplasty, or, if no angioplasty was used, the stent is used to prevent elastic recoil of the artery. Stents are typically implanted via a catheter. In the case of a balloon-expandable stent, the stent is collapsed to a small diameter and slid over a balloon catheter. The catheter is then maneuvered through the patient's vasculature to the site of the lesion or the area that was recently widened. Once in position, the stent is expanded and locked in place. The stent stays in the artery permanently, holds it open, improves blood flow through the artery, and relieves symptoms (usually chest pain).
Stents are not 100% effective in preventing restenosis at the implant site. Restenosis can occur over the length of the stent and/or past the ends of the stent. Physicians have recently employed new types of stents that are coated with a thin polymer film loaded with a drug that inhibits smooth cell proliferation. The coating is applied to the stent prior to insertion into the artery using methods well known in the art, such as a solvent evaporation technique. The solvent evaporation technique entails mixing the polymer and drug in a solvent. The solution comprising polymer, drug, and solvent can then be applied to the surface of the stent by either dipping or spraying. The stent is then subjected to a drying process, during which the solvent is evaporated, and the polymeric material, with the drug dispersed therein, forms a thin film layer on the stent.
The release mechanism of the drug from the polymeric materials depends on the nature of the polymeric material and the drug to be incorporated. The drug diffuses through the polymer to the polymer-fluid interface and then into the fluid. Release can also occur through degradation of the polymeric material. The degradation of the polymeric material occurs through hydrolysis, which erodes the polymer into the fluid and hence releases the drug into the fluid as well.
An important consideration in using coated stents is the release rate of the drug from the coating. It is desirable that an effective therapeutic amount of the drug be released from the stent for the longest period of time possible. Burst release, a high release rate immediately following implantation, is undesirable and a persistent problem. While typically not harmful to the patient, a burst release “wastes” the limited supply of the drug by releasing several times the effective amount required and shortens the duration of the release period. Several techniques have been developed in an attempt to reduce burst release. For example, U.S. Pat. No. 6,258,121 B1 to Yang et al. discloses a method of altering the release rate by blending two polymers with differing release rates and incorporating them into a single layer.
Heparin, generally derived from swine intestine, is a substance that is well known for its anticoagulation ability. It is known in the art to apply a thin polymer coating loaded with heparin onto the surface of a stent using the solvent evaporation technique. For example, U.S. Pat. No. 5,837,313 to Ding et al. describes a method of preparing a heparin coating composition.
In view of the foregoing, it will be appreciated that the development of a stent having a multi-layered coating, where one layer comprises a thin film of polymeric material with a biologically active agent dispersed therein, and a second layer is disposed over the first layer where the second layer comprises a hydrophobic heparinized polymer, would be a significant advance in the art. It will also be appreciated that the current invention inhibits both restenosis and thrombosis, and can be effective in delivering a wide range of other therapeutic agents to the implant site over a relatively extended period of time.